Electrophysiological studies are being done to characterize the active and passive transport pathways of isolated epithelial tissues. In particular, studies are being done with microelectrode impalements of frog skin to assess the voltage profiles of this tissue and the distribution of the electrical resistances and EMFS between inner and outer barriers. Substances that are known to simulate and inhibit the active Na transport are being used in order to assess their effects on the transport parameters of the cells with particular reference to the current-voltage characteristics of epithelia and the process of electrical rectification described previously. BIBLIOGRAPHIC REFERENCES: O'Neil, R.G., S.I. Helman. Influence of vasopressin and amiloride on the shunt pathway of frog skin. Am. J. Physiol. 231:164-173, l976. Helman, S.I., R.S. Fisher. Stratum corneum of frog skin: Inferences for studies of Na entry and transport pool. (In press, Am. J. Physiol.: Cell Physiol., Jan., l977).